Many satellite and terrestrial optical communication systems require transmission of analog optical signals. A straightforward way to address this need is to modulate the amplitude (AM) of an optical carrier. This approach, however, suffers from a poor Signal to Noise Ratio (SNR). It is well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with AM. Pulse position modulation (PPM) is one of such techniques. In PPM, a shift in the pulse position represents a sample of the transmitted waveform, as shown in FIG. 1. It can be shown that for a given power, SNRPPM∝SNRAM(tp/τ)2, where tp is the spacing between un-modulated pulses and τ—the pulse duration, respectively. See H. S. Black, Modulation Theory, D. Van Nostrand (1953).
The implementations of PPM for optical communications require new techniques for generating trains of optical pulses whose positions are shifted in proportion to the amplitude of a transmitted waveform. Typically a bandwidth of Δf=1-10 GHz and higher is of interest for inter-satellite communications. Since pulse repetition frequencies (PRF) of 1/tp>2 Δf are required for sampling a signal of bandwidth Δf, GHz trains of picosecond (ps) pulses are required for realizing the advantages of PPM. For example, an optical inter-satellite link designed to transmit waveforms with Δf=10 GHz bandwidth requires sampling rates of PRF=1/tp≧2Δf=20 GHz. By employing 1-2 ps-long optical pulses, a 30 dB gain is realized over an AM system with equal optical power.
Trains of equally spaced optical pulses can be generated by mode-locked lasers. This is a mature technology that is currently entering commercial arena. The present inventor is also the inventor of two US patents that describe delay generators based on a chirped distributed Bragg reflector (chirped DBR) in an electro-optically-active waveguide. See U.S. Pat. Nos. 6,466,703 and 6,600,844. Such devices can be used for introducing temporal shifts to equally spaced mode-locked optical pulses and, thereby, achieve PPM. The present disclosure (as well as the second patent identified above) improves significantly the linearity of the delay generator disclosed by U.S. Pat. No. 6,466,703. The current disclosure also reduces considerably the complexity of the second approach proposed by U.S. Pat. No. 6,600,844.
U.S. Pat. No. 6,466,703 describes a delay generator based on a chirped DBR in an electro-optically-active waveguide. The delay of a reflected optical pulse is controlled by moving the reflection point in a chirped DBR structure via the electro-optic effect. This design enables large (up to hundreds of ps) temporal shifts, and such devices are being manufactured at HRL Laboratories in Malibu, Calif. Such delay generators, however, suffer from non-linearity at high frequencies, when changes in the transmitted waveform are faster than the round-trip time of an optical pulse in the devices. Such non-linearity is caused by EO-induced phase shifts experienced by already reflected (and therefore, delayed) optical pulses by subsequent changes in the transmitted waveform.
An improved design of an EO optical delay generator has been disclosed by U.S. Pat. No. 6,600,844. In this architecture, an optical pulse is reflected backward from an EO-controlled first waveguide with a DBR into a closely-coupled second waveguide, which is not affected by the EO effect. Since the reflected wave does not experience phase shifts from the voltage applied to the first waveguide, the improved EO delay generator should have much better linearity and a wider bandwidth. This improved design of a delay generator, however, is rather complicated. Moreover, one expects high optical losses in this device, since coupling between oppositely propagating optical waves in adjacent waveguides is much weaker than that between waves in a common waveguide.
This present disclosure proposes an optically-controlled delay generator, where the reflection point of signal pulses is not controlled by the EO effect, as in the two mentioned US patents, but rather by non-linear interaction between signal and control optical pulses. The proposed architecture does not suffer from the non-linearity of the first design, since reflected signal pulses do not interact efficiently with the counter-propagating control radiation. Secondly, the proposed design is less complex and more efficient than the second design, since it does not rely on evanescent coupling between two adjacent waveguides.
The following documents describe technology for making all-optical switches based on non-linear GaInAsP waveguides with uniform distributed Bragg reflectors (DBR):    K. Nakatsuhara, T. Mizumoto, R. Munakata, Y. Kigure, and Y. Naito, “All-Optical Set-Reset Operation in a Distributed Feedback GaInAsP Waveguide”, IEEE Phot. Tech. Lett., vol. 10(1), 1998, pp. 78-80, the disclosure of which is hereby incorporated herein by reference.    K. Nakatsuhara, T. Mizumoto, E. Takahashi, S. Hossain, Y. Saka, B.-J. Ma, and Y. Naito, “All-Optical Switching in a Distributed-Feedback GaInAs Waveguide”, App. Optics, vol. 38(18), 1999, pp. 3911-16, the disclosure of which is hereby incorporated herein by reference.    S.-H. Jeong, H.-C. Kim, T. Mizumoto, J. Wiedmann, S. Arai, M. Takenaka, and Y. Naito, “Polarization Insensitive Deep-Ridge Vertical-Groove DFB Waveguide for All-Optical Switching”, Electron. Lett., vol. 37(23), 2001, pp. 1387-9, the disclosure of which is hereby incorporated herein by reference.    S.-H. Jeong, H.-C. Kim, T. Mizumoto, J. Wiedmann, S. Arai, M. Takenaka, and Y. Naito, “Polarization Independent All-Optical Switching in a Non-Linear GaInAsP-lnP Highmesa Waveguide with a Vertically Etched Bragg Reflector”, IEEE J. Quant. Electron., vol. 38(7), 2002, pp. 706-15, the disclosure of which is hereby incorporated herein by reference.
I propose to employ the same technology for making optically-controlled delay lines based on non-linear waveguides with chirped DBRs.